β-Endorphin inhibits phagocytic activity of lizard splenic phagocytes through μ receptor-coupled adenylate cyclase-protein kinase A signaling pathway

The receptor-coupled intracellular signaling mechanism of endogenous opioid peptide beta-endorphin (beta-end) is explored for the first time in ectothermic vertebrates using wall lizard as a model. beta-End inhibited the percentage phagocytosis and phagocytic index of lizard splenic phagocytes in a dose-dependent manner. The inhibitory effect of beta-end on phagocytosis was completely antagonized by non-selective opioid receptor antagonist naltrexone and also by selective mu-receptor antagonist CTAP. However, selective antagonists for other opioid receptors like NTI for delta-receptor and NorBNI for kappa-receptor did not alter the effect of beta-end on phagocytosis. This suggests that beta-end mediated its inhibitory effect on phagocytic activity of splenic phagocytes exclusively through mu opioid receptors. The mu opioid receptor-coupled downstream signaling cascade was subsequently explored using inhibitors of adenylate cyclase (SQ 22536) and protein kinase A (H-89). Both SQ 22536 and H-89 abolished the inhibitory effect of beta-end on phagocytosis in a concentration-related manner. Implication of cAMP as second messenger was corroborated by cAMP assay where an increase in intracellular cAMP level was observed in response to beta-end treatment. It can be concluded that beta-end downregulated the phagocytic activity of lizard splenic phagocytes through mu opioid receptor-coupled adenylate cyclase-cAMP-protein kinase A pathway. (C) 2011 Elsevier Inc. All rights reserved.